Thermal ink-jet printers offer a low cost, high quality, and comparatively noise-free option to other types of printers commonly used with computers. Such printers employ a resistor element in a chamber provided with an egress for ink to enter from a plenum. The plenum is connected to a reservoir for storing the ink. A plurality of such resistor elements are arranged in a particular pattern, called a primitive, in a printhead. Each resistor element is associated with a nozzle in a nozzle plate, through which ink is expelled toward a print medium. The entire assembly of printhead and reservoir comprise an ink-jet pen.
In operation, each resistor element is connected via a conductive trace to a microprocessor, where current-carrying signals cause one or more selected elements to heat up. The heating creates a bubble of ink in the chamber, which is expelled through the nozzle toward the print medium. In this way, firing of a plurality of such resistor elements in a particular order in a given primitive forms alphanumeric characters, performs area-fill, and provides other print capabilities on the medium.
Many inks that are described for use in ink-jet printing are usually associated with non-thermal ink-jet printing. An example of such non-thermal ink-jet printing is piezoelectric ink-jet printing, which employs a piezoelectric element to expel droplets of ink to the medium. Inks suitably employed in such non-thermal applications often cannot be used in thermal ink-jet printing, due to the effect of heating on the ink composition.
Paper cockle remains a problem with certain thermal ink-jet inks printed on paper passing over either a heated or non-heated platen. Specifically, during printing a full page image (0.25 inch margin on three or four sides of an 81/2.times.11 page), with certain inks, the surface of the paper puckers or crinkles, generating a washboard-like surface texture. This phenomenon, called "cockle", occurs during printing, and can be severe enough to interfere with the printing process. This is to be contrasted with paper curl, in which the paper turns into a scroll-like tube. The onset of curling sometimes takes days to develop.
A need exists for a reformulation of certain thermal ink-jet inks to avoid paper cockle.